This invention relates to dynamic hologram recording. A dynamic hologram recording device is a kind of optical four-wave mixer. In its `recording` mode two optical waves are caused to interfere in the device so as to produce a modification of its optical properties and form a (temporary) holographic record of the interference pattern. In its `playback` mode a third wave, not necessarily of the same frequency, is arranged to be incident upon the device where it interacts with the recorded hologram to produce a diffracted fourth wave.
One example of a four wave mixer is provided by the bismuth silicon oxide mixer in which the recording is effected electronically insofar as the recording results from the local trapping of photo carriers in the crystal medium. An alternative form of mixer in which the recording results from thermal effects is described in the specification of U.S. patent application Ser. No. 697,208 filed Feb. 1, 1985, now abandoned. A description of this alternative form of mixer has subsequently been published (Nov. 13, 1985) in United Kingdom patent specification No. GB 2154024A to which attention is directed. The present invention is concerned with modifications to the method of providing optical four-wave mixing that is described in that patent specification.